When you switch devices, usually everything should transfer automatically.

If you use cloud calendars, all data should transfer from the cloud to the new device, once the cloud services are set up (depending on the phone manufacturer this ideally works automatically, otherwise see Cloud Synchronization).

If you use local calendar, you should already have a backup solution in place, then you can just transfer the calendar data from the PC to the new device.

Otherwise see using local data for how to sync your data to a PC and then from there to the new device (or better import into a cloud service).